The invention concerns acoustic cavitation, but more specifically, the invention concerns a method of and an apparatus for suppressing cavitation on a surface of an element in a mechanical system, such as working surfaces in hydraulic equipment.
Cavitation concentrates energy; its occurrence may deleteriously erode surfaces of mechanical elements, even elements made of tungsten or steel. Problematic cavitation is encountered in land, sea, and air vehicles or equipment, whose designs are typically made to avoid cavitation events. At least one hundred years have passed since initial studies of propeller erosion, and since then, much knowledge has been gained in the industry about bubble dynamics and the energetics of cavitation. But even today suppressing cavitation has not successfully been achieved; it still remains an engineering priority. Developing cavitation-proof structural designs and materials often leads to extremely conservative designs, invariably underrealizing the full performance potential of many systems. Cavitation also limits the amount of power that may be mechanically coupled to fluids in hydraulic systems. Fluid includes liquids or other matter in which cavitation may be invoked.
A known method to reduce cavitation includes statically over-pressuring a region over a surface where cavitation pre-emption is sought. As known in the art, subjecting an imperfectly wetted crevice-like region located on surfaces or liquid-borne particles to reduce pressure or tensile environments may nucleate cavitation. Imposing a sufficiently high static pressure on the liquid drives a crevice's liquid meniscus to its root. Eventually, the liquid may adequately wet the crevice-like feature and thus preempt cavitation. It is extremely difficult, however, to create cavitation in fully wetted regions associated with liquids because homogeneous nucleation thresholds generally exceed several hundred atmospheres of peak negative pressure. If, however, overpressuring does not fully wet all cavitation nuclei, should cavitation occur, the violence of cavitation implosion in the statically overpressured environment would be more energetic thereby causing greater surface damage, thus defeating the purpose of cavitation-proof design in the first place.
The above-mentioned hyperbaric confinement technique of cavitation suppression requires containment of the region if it is to be made cavitation-proof. Such physical containment constitutes a primary limitation to cavitation suppression on exterior surfaces of a mechanical element or structure, like a sonar dome or propeller of a sea vessel.
Thus, it is highly desirable to provide a method of and system for cavitation suppression operable in either contained or open environments, and specifically, to provide a method and a system that does not require any hyperbaric confinement.
The confinement limitation may be circumvented by subjecting the targeted region to become a “cavitation proof surface” (CPS) with high frequency ultrasonic waves. Inventor A. Ruffa of U.S. Pat. Nos. 5,996,630; 5,884,650; and 5,717,657 has suggested using a high frequency acoustic energy field, of say one MHz, to flow over the intended CPS. As is known in the art and as Ruffa indicates that such insonification will prevent cavitation occurrences because cavitation thresholds are high and the time duration available for bubble processes is small at high frequencies. However, it has found that one can indeed bring about cavitation quite readily at high frequencies upon invoking an effect known as “acoustic coaxing” to facilitate cavitation nucleation. Implementing Ruffa's suggestion does not appear operable to suppress cavitation, but in fact is believed to enhance cavitation.